This invention relates generally to a method and apparatus for estimating the signal-to-noise ratio in communications systems.
In all communication systems, there are built-in or required performance evaluation or fault detection techniques. The signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d) of the received signal is the ultimate sought after figure of merit for receiver decisions. However, for conventional single carrier systems, there is no way of calculating the SNR without using bandwidth consuming pilot symbols or tones. Therefore, most of these systems rely instead upon a less desirable metricxe2x80x94namely, the total received signal plus noise power.
The most common use of the SNR in communication systems is in conjunction with diversity combining techniques. It is well known that the optimum diversity combining technique is Maximal Ratio Combining (xe2x80x9cMRCxe2x80x9d). During MRC, different copies of the same information are combined after being weighted by the ratio of the instantaneous signal voltage and the noise power of each copy. See, e.g., W. C. Jakes, Microwave Mobile Communications, pp. 399-423, IEEE Press, 1974. In the experimental system described therein, the SNR is estimated by measuring the signal power when the transmitter is turned on and the noise power when the transmitter is turned off. However, this is particularly cumbersome. In a recent personal communication system (xe2x80x9cPCSxe2x80x9d) standard (namely, IS-95-A), MRC is performed using a Rake receiver by weighting the output of each of the fingers of the receiver by the output of the respective pilot fingers. However, even while performing the pilot-aided coherent combining dictated by the standard, the actual weights are not a function of the SNR, but rather are a function of the signal strength plus noise.
The above-identified problems are solved and a technical advance is achieved in the art by providing a method and apparatus for estimating the signal-to-noise ratio (SNR) in OFDM communication systems. An exemplary method for determining a signal-to-noise ratio in a communication system includes receiving an OFDM symbol comprising a plurality of active sub-carriers and a plurality of inactive sub-carriers; determining a noise power of the inactive sub-carriers and a signal plus noise power of the active sub-carriers; subtracting the noise power from the signal plus noise power to obtain the signal power; and dividing the signal power by the noise power to obtain the SNR.
As previously discussed, the SNR is of critical importance in diversity combining techniques. However, it can also be used in many other applications. For example, it can be used to guide a receiver in making important decisions such as the declaration of carrier lock, system acquisition, data mode failure and the like. In accordance with the present invention, the SNR can now be easily and accurately estimated for use in diversity combining and other applications. Moreover, unlike conventional performance evaluation techniques, the present SNR estimation method does not require any channel sounding and thus, does not require the transmission of special signals such as pilot symbols or pilot tones.
Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the attached drawings.